As is well-known, the catalytic conversion of virgin gas oils containing aromatic, naphthenic and paraffinic molecules results in the formation of a variety of distillates that have ever-increasing utility and importance in the petrochemical industry. One potential use for such distillates is in the manufacture of carbon artifacts. As is well-known, carbon artifacts have been made by pyrolyzing a wide variety of organic materials. Indeed, one carbon artifact of particularly important commercial interest is carbon fiber and particular reference is made herein to carbon fiber technology. Nevertheless, it should be appreciated that this invention has applicability to carbon artifacts in a general sense, with emphasis upon the production on shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets, etc.
The use of carbon fibers for reinforcing plastic and metal matrices has gained considerable commercial acceptance. The exceptional properties of these reinforcing composite materials, such as their high strength to weight ratio, clearly offset their high preparation costs. It is generally accepted that large scale use of carbon fibers as reinforcing material would gain even greater acceptance in the marketplace, if the cost of the fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many materials containing polycondensed aromatics can be converted at early stages of carbonization to a structurally ordered optically anisotropic spherical liquid crystal called mesophase. The presence of this ordered structure prior to carbonization is considered to be fundamental in obtaining a high quality carbon fiber. Thus, one of the first requirements of a feedstock material suitable for carbon fiber production, is its ability to be converted to a highly optically anisotropic material.
In addition, suitable feedstocks for carbon artifact manufacture, and in particular carbon fiber manufacture, should have relatively low softening points and sufficient viscosity suitable for shaping and spinning into desirable articles and fibers.
Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke, infusible materials, and/or high softening point components, are detrimental to the fiber-making process.
Another important characteristic of the feedstock for carbon artifact manufacutre is its rate of conversion to a suitable optically anisotropic material.
U.S. Pat. No. 4,208,267 teaches that typical grafitized carbonaceous pitches contain a separable fraction which has important physical and chemical properties, exhibiting a softening range viscosity suitable for spinning and having the ability to be converted rapidly to an optically anisotropic, deformable, liquid crystalline material structure. Unfortunately, the amount of separable fraction present in well-known commercially available petroleum pitches, such as Ashland 240 and Ashland 260, to mention a few, is exceedingly low. For example, no more than about 10% of the Ashland 240 pitch constitutes a separable fraction capable of being thermally converted to a deformable anisotropic phase. U.S. Pat. No. 4,184,942 teaches that the amount of the aforementioned fraction can be increased by heat soaking the feedstock at temperatures in the 350.degree.-450.degree. C. until sphericals visible under polarized light begin to appear.
In U.S. Pat. No. 4,271,006, a process has been disclosed for converting catalytic cracker bottoms of a feedstock suitable in carbon artifact manufacture which requires stripping the cat cracker bottoms of fractions boiling below 400.degree. C. and thereafter heat soaking followed by vacuum stripping to provide a carbonaceous aromatic pitch.
The heavy aromatic residues used in carbon artifact manufacture are produced as by-products from the thermal or catalytic cracking of petroleum and coal feedstocks. Examples are the cat cracker bottom obtained from the catalytic cracking of petroleum distillate, the steam cracker tar produced from the steam cracking of naphtha or gas oil, and the coal tars from coal carbonization, liquefaction, or gasification. These heavy aromatic residues vary in their chemical structure, molecular weight, aromatic ring distributions and thermal and coking characteristics as a result of differences in the feed, the process and the conditions used for processing the feed. A summary of characteristics of various heavy aromatic residues is set forth in Table I:
TABLE I __________________________________________________________________________ THE CHEMICAL CHARACTERISTICS OF HEAVY AROMATIC FEEDSTOCK __________________________________________________________________________ CAT CAT STEAM STEAM FLUID CAT HEAVY CRACKER CRACKER CRACKER CRACKER CRACKER AROMATIC BOTTOM BOTTOM TAR TAR COAL COAL BOTTOM FEEDSTOCK (CCB) (CCB) (SCT) (SCT) TAR (CT) TAR (CT) (FCCB) __________________________________________________________________________ LOW HIGH STEAM HIGH TEMP- SEVERITY SEVERITY CRACKING STEAM ERATURE COAL FLUID PROCESS OF CATALYST CATALYST OF CRACKING COAL CAR- LIQUE- CATALYTIC PRODUCTION CRACKING CRACKING GAS OIL (NAPHTHA) BONIZATION FACTION CRACKING __________________________________________________________________________ AROMATICITY 33 65 70 72 90 57 33 (AROMATIC CARBON, ATOM %) (CARBON-CMR) AROMATIC PROTONS 13 27 44 46 55 21 10 (%) (PROTON-NMR) COKING YIELD AT 6 10 20 8 6 15 7 (550.degree. C.) (%) (5MTTP METHOD PI-10-67) AVE. MOL. WEIGHT 240 260 300 310 220 210 370 (GPC METHOD) ASPHALTENE (%) 1 1-5 20-30 5-15 2 10 1 (n-HEPTANE INSOLUBLES) C/H ATOMIC RATIO 0.80 0.96 1.05 1.02 1.50 1.30 0.81 __________________________________________________________________________
The heavy aromatic residues are composed of two components: (1) a low molecular weight oil fraction which can be distilled; and (2) an undistillable fraction of high molecular weight. The high molecular weight fraction is insoluble in paraffinic solvents such as n-heptane, iso-octane, petroleum ether, etc. and is termed "asphaltene". Table II below gives the average molecular weight, carbon/hydrogen atomic ratio and the coking characteristics of the oil and asphaltene fractions of three heavy aromatic residue feedstocks.
TABLE 2 __________________________________________________________________________ CHARACTERISTICS OF ASPHALTENES AND OIL FRACTIONS IN HEAVY AROMATIC FEEDSTOCK STEAM CRACKER TAR CAT CRACKER TAR COAL TAR TOTAL ASPHAL- TOTAL ASPHAL- TOTAL ASPHAL- CHARACTERISTICS FEED TENE OIL FEED TENE OIL FEED TENE OIL __________________________________________________________________________ CARBON/HYDROGEN 1.05 1.05 1.05 1.05 1.26 0.94 1.33 1.41 1.27 ATOMIC RATIO AVERAGE MOL. 280 700 180 180 650 180 185 220 150 WEIGHT (Mn) COKING VALUE 20 45 7 7 65 4 6 13 NIL (WT %) at 550.degree. C. __________________________________________________________________________
In addition to varying in chemical structure, molecular weight and coking characteristics, the oil and asphaltene vary significantly in their boiling and thermal characteristics. FIG. 1 illustrates the differential thermogravimetric analysis of a steam cracker tar heavy aromatic residue and its oil and asphaltene fractions. As can be seen, the two fractions have different boiling and decomposition ranges.
The two parts of the heavy residue have varying aromatic ring distribution. The oil fraction is composed of 2, 3, 4, 5 and 6 polycondensed aromatic rings. The asphaltene fraction is composed of 7 or more polycondensed aromatic rings.
The oil and the asphaltene parts vary significantly in their molecular weights, the oil being of a low average molecular weight of 200-250 while the asphaltene has a very broad and much higher molecular weight (Mn=600-1500). For example, the molecular weight distribution of a steam cracker tar asphaltene is illustrated in FIG. 2.
Table 3 below, illustrates the differences in chemical, physical, coking, thermal and molecular weight characteristics of the asphaltene and the deasphaltenated oils of a steam cracker tar pitch, a coal tar pitch and a petroleum pitch.
TABLE 3 __________________________________________________________________________ CHARACTERISTICS OF SCT-PITCH, COAL TAR PITCH, PETROLEUM PITCH, THEIR C.sub.7 ASPHALTENES AND DEASPHALTENATED OILS (DAO) PITCH TYPE SCT-PITCH (CP15) COAL TAR PITCH PETROLEUM PITCH TOTAL ASPHAL- TOTAL ASPHAL- TOTAL ASPHAL- PITCH TENE DAO PITCH TENE DAO PITCH TENE DAO __________________________________________________________________________ FRACTION (WT. %) 100 68.0 32.0 100 77.0 23.0 100 70.0 30.0 COKING VALUE 54.0 76.5 12.0 59.7 75.0 10.7 54.0 68.5 17.8 @ 550.degree. C. (WT. %) BENZENE INSOLUBLES 29.1 48.0 0.04 39.0 56.0 0.01 6.0 10.0 0.01 (WT. %) AROMATIC CARBON 78 76 74 92 91 90 82 81 78 (ATOM %) CARBON/HYDROGEN 1.38 1.42 1.16 1.60 1.69 1.45 1.40 1.39 1.21 ATOMIC RATIO COKING VALUE @ -- 51.7 3.8 -- 57.7 2.5 -- 47.9 5.3 550.degree. C. (%) CONTRIBUTED BY FRACTION BENZENE INSOLUBLES -- 32.6 NIL -- 43.1 NIL -- 7.0 NIL (%) CONTRIBUTED BY FRACTION __________________________________________________________________________
The asphaltenes present in aromatic pitch have higher coking and molecular weights as illustrated in Table 4:
TABLE 4 ______________________________________ MOLECULAR WEIGHT DISTRIBUTION OF SCT-PITCH, PETROLEUM PITCHES AND THEIR ASPHALTENES MOLECULAR FRACTION SCT-PITCH PETROLEUM PITCH ASPHAL- ASPHAL- MOL. WT. PITCH TENE PITCH TENE ______________________________________ 190 5.4 3.3 8.4 4.8 220 7.4 4.8 10.9 6.0 260 10.0 6.5 12.4 6.7 300 12.5 8.6 12.1 7.6 350 12.8 10.4 10.9 8.4 430 10.7 10.8 8.2 8.8 500 9.6 11.4 7.3 9.4 600 8.9 11.9 6.1 9.6 720 7.2 10.9 4.9 9.0 890 4.5 7.5 3.7 7.5 1060 2.8 4.8 2.8 6.1 1290 1.7 3.0 2.0 4.6 1560 0.9 1.7 1.3 3.2 1920 0.4 0.8 0.7 1.9 2400 0.2 0.3 0.4 1.0 No. Ave. Mol. Wt. 444 635 463 563 Calculated Peak Ave. Mol. 560 845 544 660 Wt. ______________________________________
The use of heavy residues containing oil and asphaltene fractions is not desirable for mesophase pitch production. The presence of the two components in the feed which vary significantly in their chemical, thermal, coking and molecular weight makes it difficult to define process conditions suitable for the polymerization/aromatic ring condensation of both the oil and the asphaltene parts of the feed. To overcome this problem, the heavy residue fraction has been treated to remove the asphaltene fraction by means of a distillation or a solvent deasphaltenation and the resulting asphaltene free fraction is thereafter employed. For example, in Application U.S. Ser. No. 291,990 (filed Aug. 11, 1981 and assigned to a common assignee), a process is described for heat soaking a deasphaltenated cat cracker bottom.
The separated asphaltene fraction constitutes a waste product and it would be desirable to be able to convert the asphaltene into a carbon artifact. It has recently been reported that a coal derived asphaltene, a waste obtained when coal is converted into liquid fuel, can be used to manufacture a carbon fiber composite (citation).
The present invention uses asphaltene feedstock fractions to provide a pitch which can be converted into a carbon artifact. The aromatic asphaltene fractions, because of their high molecular weight and high coking tendency, must be processed under specific certain conditions. If they are not so processed, the asphaltene will be converted into isotropic coke which is not useful for fabrication into anisotropic carbon products.